Computed Tomography (CT) systems and related types of radiographic volume imaging apparatus are known and widely used in various medical and dental applications. See for example U.S. Pat. No. 6,275,562 (He) or U.S. Pat. No. 5,867,555 (Popescu). As illustrated in FIG. 1, a CT imaging apparatus 10 includes an x-ray source 22 that emits an x-ray beam toward one or more detectors 24. In this figure, the x-ray source 22 emits a fan-shaped beam and the detectors 24 are a bank of detectors. Other types of beam shapes (for example, cone beam) and detector shapes (for example, a flat panel detector array) are known. In the system shown in FIG. 1, both source 22 and detector 24 are mounted on a rotational gantry 12 that is actuable to rotate about patient 14. The beam irradiates a slice of patient 14 and the resulting signals at detector 24 channels are sampled by a data measurement system to form a projection data set. Other configurations for CT imaging systems are known, including a number of variants of the basic model shown in FIG. 1. These can include cone-beam CT (CBCT) and multidetector computed tomography (MDCT) apparatus.
Medical x-rays are a valuable tool for diagnosing and treating disease. Along with its benefits, however, there are risks, since the radiation generated by the x-ray system may pose a risk of cancer. To help reduce radiation exposure to the patient, it is desired to acquire diagnostic radiographic images with a low dose, preferably at the lowest possible dose that results in an acceptable diagnostic image.
The relative thickness dimensions and density of the subject anatomy are factors that determine how much radiation energy is needed to obtain suitable image quality from detector 24 data in a radiographic volume imaging apparatus. Across the patient population, there is significant variation in the human body shape, depending on factors such as patient age, sex, height, and weight, for example. In addition, the human body does not conform to ideal geometric shapes or to uniform density or absorption characteristics; there can be significant cross-sectional variation from a cylindrical shape over the patient population. In addition, there are also vast disparities in anatomy density among different body parts (e.g., head vs. chest). Thus, for different patients or body parts, the absorption, scattering, and attenuation of the x-ray beam, and therefore the quantum noise, will vary significantly over a wide range, leading to inhomogeneous noise in the final reconstructed CT images.
Approaches to reducing exposure where possible, based on patient anatomical features include automatic tube current modulation (ATCM) and automatic exposure control (AEC) systems. ATCM and AEC systems have been used with some CT scanners to achieve lower patient dose by adaptively modulating the tube current in the x-y plane (angular modulation) or along the z-axis (z-axis modulation) according to size and attenuation characteristics of the body part being imaged. However, in order to compute tube current at different angles, these methods require obtaining two initial x-ray scout images from orthogonal directions, correspondingly defeating the purpose of ATCM by increasing patient dose. Moreover, existing approaches fail to take into account actual dimensional characteristics of the subject and typically assume that the scanned body part has a uniformly symmetric shape and contour (e.g., circular or elliptical).
The discussion above is merely provided for general background information on some of the problems addressed and is not intended to be used as an aid in determining the scope of the claimed subject matter. The invention is defined by the claims.